Some colleges have some kind of committee that exercises a gatekeeper role on medical school applications – the college “supports” (or, conversely, fails to support) the student’s medical school applications. They do this because it looks a whole lot better to say “85% of the students (including alumni) who apply to U.S. medical school with the support of the college are accepted to at least one U.S. medical school” than to say “45% of our students or alumni who apply to U.S. medical school are successful.” Students (and recent alumni) who do not get that “support” at colleges that offer it can be at a terrible disadvantage. So the OP’s child may want to understand what it will take to have a supported application if he or she attends that sort of college.
I agree with everyone else who has said start now. But the student really doesn’t have to get into a position to apply to medical school for the year following college graduation, or even the year after that. It will take some time to build an appropriate science/med resume, and to prepare carefully for the MCATs, so the possibility of going straight from college to medical school has probably already passed as a practical matter. It may make sense to plan on completing the last one or two required pre-med courses as a non-degree student the year after graduation, while working part-time in some appropriate job, then taking the MCATs a year after graduation and applying the following fall. Doing that also hedges the bets a little bit for the rest of college. The student doesn’t have to fill a schedule completely with basic science courses and labs only to find that, yes, the student still doesn’t actually like science!
(One of my sisters decided at 33, in the middle of a life crisis, that she had always wanted to go to medical school, she had just forgotten to take any science courses after 10th grade. She had been a B+ history major at a middle-of-the-road public flagship, she had a certificate in art history from an Italian university, and she had a decade of successful jobs at a regional stock exchange. It took her three years to get into medical school: two years to take the pre-med courses at a brutally competitive public flagship while working part-time in a medical research lab and shadowing doctors, then MCATs, then an academic year’s worth of application process while continuing her lab job. A lifelong academic underachiever to that point, she understood that she had no margin for error. She worked her tail off and got As or A+s in all of the premed requirements, and did very well on the MCATs. It helped a lot that she did this in a community where she had lots of ties to doctors and medical researchers who really joined the team to get her into medical school at age 36. She was 44 when she finished her residency.
She’s a great doctor.)